The present invention relates to a method for producing a solar cell utilizing a silicon single crystal wafer useful as a material of solar cell and a solar cell.
When a silicon single crystal is used as a material for producing a solar cell, reduction of the production cost as well as improvement of the conversion efficiency have constituted serious problems.
Hereafter, the technical background of use of a silicon single crystal as a material for solar cells will be explained.
Characteristics of solar cells will be explained first based on type of a substrate material constituting a solar cell. Solar cells are roughly classified based on the type of the substrate material into three types, i.e., xe2x80x9csilicon crystal type solar cellsxe2x80x9d, xe2x80x9camorphous silicon type solar cellsxe2x80x9d and xe2x80x9ccompound semiconductor type solar cellsxe2x80x9d, and the silicon crystal type solar cells further include xe2x80x9csingle crystal type solar cellsxe2x80x9d and xe2x80x9cpolycrystal type solar cellsxe2x80x9d. Among these, the solar cells showing high conversion efficiency, which is the most important characteristic as a solar cell, are the xe2x80x9ccompound semiconductor type solar cellsxe2x80x9d, and the conversion efficiency thereof reaches almost 25%. However, as for the compound semiconductor type solar cells, it is extremely difficult to produce compound semiconductors used as the materials thereof, thus they have a problem for becoming popular for general use in respect of the production cost of solar cell substrates, and use thereof has been limited.
The term xe2x80x9cconversion efficiencyxe2x80x9d used herein means a value representing xe2x80x9ca ratio of energy, which can be converted into electric energy by a solar cell taken out of the solar cell, to energy of light irradiated on the solar cellxe2x80x9d and represented in percentage (%) (it is also called photoelectric conversion efficiency).
Solar cells showing high conversion efficiency in the next place to the compound semiconductor type solar cells are silicon single crystal type solar cells. Since they show power generation efficiency of 20% order, which is close to that of the compound semiconductor solar cells, and substrates for those solar cells may be relatively easily obtained, they constitute the mainstream of the solar cells for wide general use. Furthermore, the silicon polycrystal type solar cells and amorphous silicon type solar cells are also practically used because of low production cost of the solar cell substrate materials therefor, although the conversion efficiencies thereof are inferior to those of the aforementioned two types of solar cells, i.e., about 5 to 15%.
A general method for producing a silicon single crystal type solar cell will be briefly explained hereafter. First, a cylindrical ingot of silicon single crystal is produced by the Czochralski method (referred to as the CZ method or the Czochralski method hereafter) or the floating zone melting method (referred to as the FZ method or the floating zone method hereafter) in order to obtain a silicon wafer serving as a substrate of a solar cell. Further, this ingot is sliced into, for example, a thin wafer having a thickness of about 300 xcexcm, and the wafer is etched for the surface with a chemical solution to remove mechanical damages on the surface to obtain a wafer (substrate) used as a solar cell. This wafer is subjected to a diffusion treatment for impurity (dopant) to form a pn-junction on one side of the wafer, electrodes are attached on the both sides, and an antireflection film for reducing light energy loss by light reflection is finally attached on the sunlight incidence side surface to complete a solar cell.
Although demands for solar cells are recently increasing as one of clean energy sources with a background of environmental problems, the higher energy cost compared with general commercial powers has constituted an obstacle of wide use thereof. In order to reduce the cost of silicon crystal solar cells, it is necessary to further increase the conversion efficiency as well as to reduce the production cost of substrates. For this reason, the cost of substrate material has been reduced by using cone portions, tail portions of single crystal ingots and so forth, which can not be made into products or are not suitable for electronic use for producing so-called semiconductor devices, as raw materials of substrates of single crystal type solar cells. However, such supply of raw materials is unreliable, and the amount thereof is also limited. Therefore, considering expansion of the demands for silicon single crystal type solar cells in future, it will be difficult to stably produce solar cell substrates in a required amount by such a method.
Moreover, in solar cells, it is important to produce a solar cell of larger area in order to obtain a larger electric current. As a method of obtaining a silicon wafer having a large diameter used as a substrate material for producing a solar cell of large area, the CZ method is suitable, which enables easy production of a silicon single crystal having a large diameter and provides superior strength of a produced single crystal. Therefore, the CZ method constitutes the mainstream of the production of silicon crystals for solar cells.
Further, if a silicon wafer serving as a substrate material of a single crystal type solar cell does not have a substrate lifetime (referred to as xe2x80x9clifetimexe2x80x9d or abbreviated as LT hereafter), which is one of the characteristics thereof, of 10 xcexcs or more, it cannot be used as a solar cell substrate. Furthermore, in order to obtain a solar cell of high conversion efficiency, it is required that the substrate lifetime should be preferably 200 xcexcs or more.
However, as for a single crystal produced by the CZ method, which is the mainstream of the current methods for producing single crystal ingots, if the solar cell is irradiated with a strong light when the single crystal is processed into a solar cell, lifetime of the solar cell substrate is reduced, and photodegradation is caused. Therefore, sufficient conversion efficiency cannot be obtained, and improvement is desired also for performance of solar cells.
It is known that the cause of the reduction of the lifetime and the photodegradation upon irradiation of strong light on a solar cell produced by using such a CZ method silicon single crystal is an influence of boron and oxygen present in the single crystal substrate. Currently, the conductivity type of wafers used as solar cells is mainly p-type, and boron is usually added to p-type wafers as a dopant. Although a single crystal ingot used as the material of the wafer may be produced by either the CZ method (including the magnetic field applied CZ method, also referred to as the MCZ method hereinafter) or the FZ method, the FZ method suffers from higher production cost for single crystal ingots compared with the CZ method, and in addition, a silicon single crystal having a large diameter is more easily produced by the CZ method as described above. Therefore, at present, single crystals are mainly produced by the CZ method, which enables production of single crystals having a large diameter at a relatively low cost.
However, a crystal produced by the CZ method suffers from a problem that it contains oxygen at a high concentration, and thus the lifetime characteristic is affected by boron and oxygen in a p-type CZ-method silicon single crystal to cause photodegradation.
In order to solve such a problem, the applicants of the present application proposed use of Ga (gallium) instead of B (boron) as a p-type doping agent in a previous application (PCT/00/02850). By using Ga as a dopant as described above, it became possible to prevent reduction of the lifetime due to the influence of B and oxygen.
However, in spite of the elimination of the influences of B and oxygen by use of Ga as the dopant, the lifetime might be reduced and characteristics of solar cells fluctuated among produced solar cells. Such fluctuation of characteristics invited decrease of production yield of solar cells and decrease of the conversion efficiency as the whole solar cell module and thus caused a problem.
The present invention was accomplished in view of such a problem, and its object is, when a solar cell is produced by using a CZ silicon single crystal wafer, to provide a solar cell showing little fluctuation of characteristics by using a CZ silicon single crystal wafer that does not reduce the lifetime.
In order to solve the aforementioned problem, the present invention provides a method for producing a solar cell comprising forming the solar cell from a CZ silicon single crystal wafer, wherein a CZ silicon single crystal wafer having an initial interstitial oxygen concentration of 15 ppma or less is used as the silicon single crystal wafer.
If the initial interstitial oxygen concentration is 15 ppma (JEIDA: Japan Electronic Industry Development Association Standard) or less as described above, oxygen precipitation is hardly generated by a heat treatment for producing a solar cell, a solar cell that avoids reduction of lifetime by BMD can be obtained, and thus a favorable solar cell showing little fluctuation of characteristics can be produced.
In the aforementioned method, the CZ silicon single crystal wafer is preferably a p-type silicon single crystal wafer containing Ga as a dopant.
By using Ga as a dopant of p-type silicon single crystal wafer instead of boron, the photodegradation caused by not only BMD but also presence of boron and oxygen can be prevented.
In the aforementioned method, the concentration of Ga is 3xc3x971015 to 5xc3x971017 atoms/cm3.
If the concentration of Ga is 3xc3x971015 atoms/cm3 or more as described above, reduction of conversion efficiency by consumption of power due to increase of internal resistance of the solar cell can be suppressed, and if the concentration of Ga is 5xc3x971017 atoms/cm3 or less as described above, the so-called Auger recombination phenomenon, i.e., reduction of the lifetime due to capture of minority carriers by Ga atoms, can be prevented.
Further, the solar cell produced by the method of the present invention is, for example, a solar cell produced from a CZ silicon single crystal wafer, wherein the CZ silicon single crystal wafer has an interstitial oxygen concentration of 15 ppma or less.
If the solar cell is produced from a CZ silicon single crystal wafer having an interstitial oxygen concentration of 15 ppma or less as described above, oxygen precipitation is hardly generated by a heat treatment for producing a solar cell, and the interstitial oxygen concentration is not so high either. Therefore, the reduction of the lifetime resulting from oxygen atoms themselves can be suppressed, and the solar cell shows little fluctuation of characteristics.
The solar cell of the present invention is also a solar cell produced from a CZ silicon single crystal wafer, wherein the CZ silicon single crystal wafer has a BMD density of 5xc3x97108/cm3 or less.
If the CZ silicon single crystal wafer has a BMD density of 5xc3x97108/cm3 or less as described above, sharp reduction of the lifetime can be prevented, the conversion efficiency of the solar cell can also be maintained at a high level, and thus a solar cell showing little fluctuation in characteristics can be provided.
In this case, the CZ silicon single crystal wafer constituting the aforementioned solar cell is preferably a p-type silicon single crystal wafer containing Ga as a dopant.
This is because, if the dopant of p-type silicon single crystal wafer is not boron, but Ga, as described above, the photodegradation caused by the presence of boron and oxygen can also be prevented.
In this case, the concentration of Ga is preferably 3xc3x971015 to 5xc3x971017 atoms/cm3.
If the concentration of Ga is 3xc3x971015 atoms/cm3 or more as described above, reduction of conversion efficiency by consumption of power due to increase of internal resistance of the solar cell can be suppressed, and if the concentration of Ga is 5xc3x971017 atoms/cm3 or less as described above, reduction of lifetime caused due to capture of minority carriers by Ga atoms, i.e., the so-called Auger recombination phenomenon, can be prevented.
As described above, according to the method for producing a solar cell and the solar cell of the present invention, a solar cell showing little fluctuation of characteristics can be obtained, and a solar cell of high efficiency can be obtained at a low cost.